Motor Overload Protection Explained

By R.W. Hurst, Editor

motor overload protection
Motor Overload Protection

Protecting your electric motor from excessive current and overheating is critical to maintaining its performance and efficiency. This is where safeguarding mechanisms come into play.

Motor overload protection is essential to maintaining performance and safety. By understanding the importance of overcurrent protection measures and the various devices available, you can make informed decisions to defend your devices from potential damage or failure.


The Role of Motor Overload Protection

These mechanisms are vital in defending motors from potential failure or damage. For example, when a squirrel cage motor starter is subjected to a higher current than it can handle, it can overheat, leading to a reduced lifespan or even complete failure. Appropriate devices help detect such issues early and automatically shut down the device before permanent damage occurs.


How Thermal Mechanisms Work

One common method is thermal protection. It is designed to defend devices from excessive heat caused by high current or stalled operations. Thermal devices, such as bimetallic relays, measure the temperature and disconnect power if the device exceeds a pre-determined temperature threshold. Bimetallic relays consist of two different metals with different thermal expansion rates, which causes the relay to trip when it reaches the set temperature.


Differences Between Overload and Short Circuit Measures

Overload and motor branch circuit measures are two distinct protective measures, often confused with each other. While the former addresses issues related to overheating and excessive current, the latter protects against an abrupt surge in current due to a fault in the wiring or other components. It also serves to protect against phase overload conditions and protect the motor from damage. Overload devices include relays and switches, while short circuit measures rely on fuses or circuit breakers.


Selecting and Sizing Devices

Proper selection and sizing of devices are crucial to ensure performance and safety. Consider the full load current, voltage, and power ratings to choose the right device. Relays should be sized according to the full load current, ensuring that the relay will trip before sustaining any damage.


The Role of Relays in Systems

Relays are essential components of protection systems. These relays detect excessive current and initiate a shutdown to defend against damage. In addition, they are typically designed with inverse time characteristics, meaning that the relay will trip faster as the current level increases. Overload relays can be electromechanical, solid-state, or bimetallic, each offering unique sensitivity, accuracy, and response time advantages.


Bimetallic Relays and Their Contribution

Bimetallic relays contribute to protection by utilizing the different thermal expansion rates of two bonded metals. When the temperature exceeds a set threshold, the bimetallic strip bends, causing the relay to trip and disconnect the power supply. This prevents overheating and ensures longevity.


Common Types of Circuits and Applications

Several circuits are designed to address various conditions, such as phase loss, ground fault, and overcurrent. Some common circuits include:

  1. Overcurrent measures: These circuits use fuses, circuit breakers, or switches to disconnect from the power supply when the current exceeds a specified threshold.
  2. Phase loss measures: This circuit monitors for a missing phase in the supply voltage, which can lead to damage if not detected and corrected promptly.
  3. Ground fault measures: This circuit detects any leakage current between the windings and the ground, which can cause failure or even fires.